WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Use a 1N4005 diode (or similar) instead. You can use the existing mounting hole for a two lug terminal strip to hold it.
The diode's band goes to the transformer lead, to create a negative voltage for the bias. Be sure to readjust the bias afterward,
since the new bias circuit will have a larger (more negative) output voltage. That's a good mono amp, do you have two?
Don't suspect things. Figure out !
You have got a link to the manual, here everything is described inkluding a schematics. Print it out, mark your changes for future references.
As for replacing the Se with Si, you might raise the bias ( make it more negative) then the pot allows for adjustment, making it impossible to properly adjust bias.
Don't panic, it's dur to the Si has less forward drop then the Se, you need to adjust
one of the resistors in the bias circuit. Here the manual comes into use.